The restless central engine of accreting supermassive black holes produces ubiquitous variability across broad ranges of wavelengths and timescales. Variability allows the study of the innermost structures around the SMBH that cannot be spatially resolved directly. In particular, reverberation mapping (RM) of the broad-line emission in response to light fluctuations from the accretion flow constrains the size, geometry and kinematics of the fast-moving gas around the SMBH, and provides a measurement of the central black hole mass.  This RM technique (and its extension) is currently our primary method to measure black hole masses in distant quasars up to z>6. After a brief review on recent progress on AGN variability, I will discuss the current status of broad-line RM, and present results from decade-long monitoring programs and applications of RM results to galaxy-SMBH co-evolution. If there is time, I will end with a "twin quasar" puzzle that has been haunting us for the past few years.

Astro Colloquium and 'coffee & cookies' department gathering (3:45-4:00pm)

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